SE-C-523 690 discloses a centrifugal separator which is intended for cleaning a gas containing liquid impurities in the form of oil particles and/or oil mist. The gas outlet for the cleaned gas extends in the known centrifugal separator from a lower part of the separation space. Above the gas outlet, there is a radial shield element which forms an annular surface extending into the separation space. Above, i.e. upstream, the shield element at a distance therefrom, there are a number of outlet holes for discharge of the separated oil from the separation space. An axial shield element extends, according to one embodiment, upwardly from the radial shield element, but is located axially beneath, i.e. axially downstream, of the outlet holes. The separated oil is caught by the inner wall of the separation space and flows downwardly towards the outlet holes. The technique disclosed in SE-C-523 690 is based on the phenomenon that the shield element will create an annular gas cushion immediately above the shield element, and that this gas cushion will stop the downwardly flowing oil at the level of the upper end of the gas cushion and at the level of the outlet holes so that the oil may be discharged through the outlet holes.
The liquid impurities, for instance oil, which deposit on or are located in a layer adjacent to the annular surface of the radial shield element, which extends into the separation space, will be conveyed radially inwardly towards the centre of the separation space along this surface since the rotation of the gas flow in this layer is decelerated by the friction to the non-rotating shield element.
The applicant has found out that a part of the downwardly flowing oil will pass the gas cushion and said shield element, which means that this part of the oil will follow the cleaned gas out from the centrifugal separator through the gas outlet.
The outlet holes of the known centrifugal separator are generally equidistantly located along the periphery of the casing. This means that the oil that is discharged through the holes has to be collected outside the casing for the transport to a common outlet.